In recent years, the methods of printing books, magazines, and other publications; handbills, pamphlets, posters, and other commercial advertising; and other printed matter have become diversified. From the viewpoint of the adaptability to single color printing and multi color printing in a broad range of applications, improvement of the opacity and absorbability of the coated paper has been sought.
In the past, a coating composition which contains hollow polymer particles has been used for production of coated paper. For example, Patent Document 1 discloses a method of producing an aqueous dispersion of hollow polymer particles which comprises taking an aqueous dispersion which contains polymer particles having a three-layer structure of a core polymer particle (A), shell layer (B), and shell layer (C) by adding a base to make the pH 7 or more and to neutralize at least part of the acidic groups which are contained in the core polymer particle (A) and then forming pores at the inside of the core polymer particle (A).
When producing an aqueous dispersion of hollow polymer particles, for removing residual monomers in the aqueous dispersion derived from the polymerization reaction, steam stripping in which steam (saturated water vapor) is blown is suitably used but in the conventionally known method of production of an aqueous dispersion of hollow polymer particles, there were also the problems that the amount of the coagulum in the aqueous dispersion of hollow polymer particles after performing the polymerization step, base treatment step, and residual monomer removal step is relatively large, the filtration time for removing this coagulum by filtration is long, so the productivity is poor, and, further, the frequency of cleaning the polymerization reactor is high, so the work efficiency is poor.